A high-voltage circuit breaker is a device used in the distribution of three-phase electrical energy in a power system. When a sensor or protective relay detects a fault or other system disturbance in connection with a protected circuit of the power system, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. A recloser differs from a circuit breaker in that a circuit breaker opens a circuit and maintains the circuit in the open position indefinitely, whereas a recloser may automatically open and reclose the circuit several times in quick succession to allow a temporary fault to clear and, thus, avoid taking the circuit out of service unnecessarily.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current-carrying contacts housed therein; the operating or driving mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high-voltage electrical energy from the protected circuit into and out of the tank(s). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Modern mechanical switches utilized in high-voltage circuit breakers comprise the interrupters and the operating mechanism which are interconnected by the mechanical linkage. The interrupters provide one or more sets of current-carrying contacts. When the switch is closed, these current-carrying contacts are electrically interfaced. As the switch is opened, an arc forms between some of the current-carrying contacts. Such arcing can cause the contacts to erode and, perhaps, to disintegrate over time. Once an arc has formed, it is extremely difficult to extinguish it until the arc current is substantially reduced. Thus, modern interrupters inject a compressed electrically insulating gas, such as SF.sub.6 into the cavity of the tank housing the interrupter to facilitate in extinguishing the arc. Once the arc is extinguished, the protected circuit is opened thereby preventing current flow.
The operating mechanism provides the necessary operating forces for opening and closing the interrupter contacts. Operating mechanisms such as a hydraulic spring type driving unit require relatively little energy to drive the interrupters open or closed and are easily adapted to store increased numbers of such operations to permit rapid and repeated operations. Moreover, independent pole operating mechanisms provide independent control for each of the three phases of the power system, i.e., each of the three interrupters can be opened or closed independently of one another.
Typically, a reactive or resistive impedance is coupled between the arcing contacts of the interrupter to control the arcing by equalizing the voltages at the respective breaks in a multi-interrupting point type circuit breaker, i.e., one with more than one set of contacts. Since resistive impedances consume real power, electric utilities prefer to interface reactive impedances with the power system.
Voltage and current transients generated during the energization of the reactive impedances have become an increasing concern for the electric utility industry in terms of power quality for voltage-sensitive loads and excessive stresses on power system equipment. For example, modern digital equipment requires a stable source of power. Moreover, computers, microwave ovens and other electronic appliances are prone to failures resulting from such transients. Even minor transients can cause the power waveform to skew, rendering these electrical devices inoperative. Therefore, utilities have set objectives to reduce the occurrence of transients and to provide a stable power waveform.
Conventional solutions for reducing the transients resulting from reactive impedance energization include circuit breaker pre-insertion devices, for example, resistors or inductors, and fixed devices such as current limiting reactors. While these solutions provide varying degrees of mitigation for reactive impedance energization transients, they result in added equipment, added cost, and can result in added reliability concerns.
For particular types of reactive impedances, the maximum transients are associated with closing the circuit breaker at the peak of the system voltage waveform. One solution to this problem is to add timing accuracy to synchronously close the circuit breaker at the instant the system voltage is substantially zero. In this way, the voltages on both sides of the mechanical switch at the instant of closure would be nearly equal, allowing for an effectively "transient-free" energization.
The reactive impedance may be capacitive or inductive. A reactive impedance connected in series with the power system is switched into the power system when the switch is opened. In contrast, the shunt reactive impedance is switched into the system when the switch is closed. The insertion or removal of the reactive impedance in the power system alters either the series impedance of the transmission or the reactive power flow in a power system. Therefore, switching the reactive impedances into and out of the power system directly or indirectly effects real power flow in the power system.
FIG. 1 shows an example of one possible power system configuration with four generators 4, 5, 6, 7. These generators supply power to loads 1, e.g., homes, factories, etc. A transmission system connected between the generators 4, 5, 6, 7 and loads 1 consists of transformers and transmission lines 2. Transmission lines 2 are primarily inductive (X) for the purposes of power flow analysis. The transmission system is interconnected by system busses 3. The generators 4, 5, 6, 7 supply real power to be consumed by loads 1 and reactive power consumed by the inductance of transmission lines and transformers.
Power oscillations in the power system may be caused by a power system disturbance such as lightning or a short circuit in the power system. Such power oscillations may lead to instability of the power system or create operating difficulties. These power oscillations are the result of interactions between generators 4-7 of the power system as an attempt is made by these generators to reach a steady-state after a power disturbance.
The time scale of the power oscillation phenomena is illustrated in FIG. 2. Each cycle of the power oscillation 8 typically has a period from about 0.5 sec. to about 5 seconds. Power system voltages and currents 9 are either about 50 or about 60 Hz corresponding to a period of approximately 20 msec per cycle or approximately 17 msec. per cycle, respectively. In other words, the power oscillation phenomena is a relatively slow phenomena compared to 60 Hz oscillations.
One approach used to combat power oscillations generated after a power disturbance is the use of Power System Stabilizers (PSS) connected directly to the power system generators. The PSS is a control system which attempts to modulate and damp the power oscillations directly at the generator by controlling parameters of the generator directly. Referring back to FIG. 1, power oscillations between generators 4 and 5 might effectively controlled by the PSS approach. However, the PSS is difficult to tune or coordinate with other PSS to provide effective damping of inter-area power oscillations, such as power oscillations between generators 4, 5 and generators 6, 7, which are separated by large distances, and thus, large inductive reactances.
An alternative means of damping power oscillations can be provided through the switching of reactive impedance one or more times during each cycle of the power oscillation at appropriate instants.
The switching of a series reactive impedance controls power flow by altering primarily the series impedance and, therefore, the current flow in the series connection of the power system. The switching of a shunt reactive impedance alters primarily the voltage at the point of connection in the power flow, thereby providing some control of power flow. The repetitive switching of reactive impedances at the appropriate points of power oscillations in the system can provide effective damping of the oscillations. Fundamentally, the shunt and series approaches provide similar power oscillation damping benefits, however, the series approach is generally regarded as more effective from a damping MV Ar perspective [L. Angquist, B. Lundin, J. Samuelsson, "Power Oscillation Damping Using Controlled Reactive Power Compensation--A Comparison Between Series and Shunt Approaches," IEEE Transactions on Power Systems, Vol. 8, No. 2, May 1993.].
Until recently, mechanical switches for switching series and shunt reactive impedances were specified only for very slow switching control, typically switching these elements only once or twice per day. Thus, damping control using mechanically switched systems required a number of redundant circuit breakers which would be sequentially opened and/or closed to damp power oscillations. Modern damping systems have failed to use the advantages of faster, more reliable, mechanical switches now available. An example of such a system has been disclosed by T. Shimojo, M. Udo, M. Masui, T. Matsushima, "Improvement Of Damping of Tie-Line Power Swing By Means Of Shunt Capacitor Switching Control," CIGRE-IFAC Symposium, Florence, Italy, 1983.
A static or solid-state switch such as a thyristor has been used for switching reactive impedances quickly for power oscillation damping (often referred to as Static V Ar Compensators (SVC) and Thyristor-Controlled Series Capacitors (TCSC), respectively). Mechanically-switched reactive impedances are sometimes included as part of these systems, however, the damping functions for power oscillations are performed completely by the solid-state or thyristor switched reactive impedances.
The use of thyristor switches has numerous practical drawbacks. For instance, commercially available thyristor switches have voltage ratings well below voltage levels in power systems in which power oscillation damping is desired. Thus, such thyristor based systems rely on the use of a voltage step-down transformer in transmission system applications adding appreciable cost to the overall switching system. Thyristor switches also have continuous real power losses which result in further costs. Moreover, thyristor switches require active cooling systems thereby additionally increasing the cost of the overall switching system. The added complexity of the overall switching system tends to degrade system reliability.
Therefore, there is a need for a dynamic mechanically-switched damping system which can provide damping control at a reduced cost, with less power loss, decreased complexity, and greater design and application flexibility. Such a damping system should include reactive impedances which may be synchronously switched at a relatively fast rate.